A horn is a sound-making device that can be equipped to motor vehicles, buses, bicycles, trains, trams (or streetcars for North American cities), and other types of vehicles. The sound made usually resembles a 'honk'. The vehicle operator uses the horn to warn others of the vehicle's approach or presence, or to call attention to some hazard. Motor vehicles, ships and trains are required by law in some countries to have horns. Bicycles like trams, trolley cars and streetcars in many areas are also legally required to have an audible warning device, but not universally, and not always a horn.

Bicycles often have a classic bulb horn, operated by squeezing a rubber bulb attached to a metal horn. Squeezing the bulb forces air through a steel reed located in the throat of the horn, making it vibrate, producing a single note. The flaring horn matches the acoustic impedance of the reed to the open air, radiating the sound waves efficiently, making the sound louder. Other types of horns used on bicycles include battery-operated horns (sometimes even car horns on 12-volt circuits are incorporated) and small air horns powered by a small can of compressed gas.

Oliver Lucas of Birmingham, England, developed a standard electric car horn in 1910. Car horns are usually electric, driven by a flat circular steel diaphragm that has an electromagnet acting on it and is attached to a contactor that repeatedly interrupts the current to that electromagnet. This arrangement works like a buzzer or electric bell and is commonly known as "sounding" or "honking" one's horn. There is usually a screw to adjust the distance/tension of the electrical contacts for best operation. A spiral exponential horn shape (sometimes called the "snail") is cast into the body of the horn, to better match the acoustical impedance of the diaphragm with open air, and thus more effectively transfer the sound energy. Sound levels of typical car horns are approximately 107–109 decibels, and they typically draw 5–6 amperes of current.

Horns can be used singly, but are often arranged in pairs to produce an interval consisting of two notes, sounded together; although this only increases the sound output by 3 decibels, the use of two differing frequencies with their beat frequencies and missing fundamental is more perceptible than the use of two horns of identical frequency, particularly in an environment with a high ambient noise level. Typical frequencies of a pair of horns of this design are 500 Hz and 405–420 Hz (approximately B4 and G♯4, minor third).

Some cars, and many motor scooters or motorcycles, now use a cheaper and smaller alternative design, which, despite retaining the name "horn," abandons the actual horn ducting and instead relies on a larger flat diaphragm to reach the required sound level. Sound levels of such horns are approximately 109–112 decibels, and they typically draw 2.5–5 amperes of current. Again, these horns can be either single, or arranged in pairs; typical frequencies for a pair are 420–440 Hz and 340–370 Hz (approximately G♯4–A4 and F4–F♯4) for this design.

Diagram showing how a car horn works

A horn grille is a part of some designs of car or other motor vehicle that has an electric horn, such as a motor scooter.

The radiators of modern cars no longer determine the shape of the grilles, which have become more abstract, the radiator being of different proportions from the grille and over 15 centimetres behind it. Now grilles are usually designed so the sound of a horn can readily come out through them. Those designs that echo the shape of the grille no longer have front fenders with rather large crevices that accommodate trumpet-shaped horns. Thus some cars, often British ones, have a pair of round horn grilles either side of the radiator grille, with a horn behind each. A luxury car's horn grilles are usually chrome-plated.

Cars with rear engines, such as the Volkswagen Beetle and the early Porsches, necessarily have no radiator grilles in front, and so have horn grilles placed below their headlights. Some motor scooters have this feature as well, placed below the handlebars. Their horn grilles may be made of cheap plastic. These vehicles and the cheaper cars have only one horn.

Truck (lorry) horns may be electrically operated and similar to car horns, but are often air horns driven by air from an air compressor, which many trucks have to operate the air brakes. The compressor forces air past a diaphragm in the horn's throat, causing it to vibrate. Such air horns are often used as trim items, with chromed straight horns mounted on top of the cab. This design may also be installed on customised automobiles, using a small electrical compressor. Usually two or more are used, some drivers go so far as to install train horns. The frequencies vary to produce a variety of different chords, but in general are lower than those of automobile horns—125–180 Hz (approximately C–G). Sound levels are approximately 117–118 decibels.

Locomotives have train horns, which are air horns operated by compressed air from the train's air brake system. To distinguish their sound from truck air horns, train horns in the U.S. consist of groups of two to five horns (called "chimes") which have different notes, sounded together to form a chord. Trains typically cannot stop in time to avoid hitting obstructions and depend on being seen by the driver, so they rely on their horns to warn of their approach. Therefore, train horns are louder and lower in frequency than car horns, so that they can be heard at longer distances. The sound level is 146–175 dB. In the United States, train horns are required to have a minimum sound level of 96 dB and a maximum sound level of 110 dB at 100 ft in front of the train.

Most modern streetcars, trams and trolley cars including low-floor vehicles around the world also employ horns or whistles as a secondary auditory warning signal in addition to the gong/bell which either use the sound of air horns or electric automobile car horns.

Ships signal to each other and to the shore with air horns (sometimes referred to as whistles) that are driven with compressed air or from steam tapped from the power plant. Low frequencies are used because they travel further than high frequencies; horns from ships have been heard as far as ten miles away.[1] Traditionally, the lower the frequency, the larger the ship. The RMS Queen Mary, an ocean liner launched in 1934, had three horns based on 55 Hz, a frequency chosen because it was low enough that the very loud sound of it would not be painful to the passengers.[2] Modern International Maritime Organization regulations specify that ships' horn frequencies be in the range 70–200 Hz for vessels that are over 200 meters in length.[3]

Portable air horns driven by canned compressed air are used for small craft water safety as well as for sports events and recreational activities.

Klaxon is a trademark for a brand of electromechanical horn or alerting device. Mainly used on cars, trains and ships, klaxon horns produce an easily identifiable sound often transcribed onomatopoeiacally in English as "awooga" or "ah-oo-gah." Like most mechanical horns, the klaxon has largely been replaced by solid-state electronic alarms, though the memorable tone itself has persisted.

The klaxon horn's characteristic sound is produced by a spring-steel diaphragm with a rivet in the center that is repeatedly struck by the teeth of a rotating cogwheel. The diaphragm is attached to a horn that acts as an acoustic transformer and controls the direction of the sound.[6]

In the first klaxons, the wheel was driven either by hand or an electric motor. American inventor Miller Reese Hutchison (later chief engineer of Thomas Edison) patented the mechanism in 1908.[7] The Lovell-McConnell Manufacturing Co. of Newark, New Jersey bought the rights to the device and it later became standard equipment on General Motors cars.[8] Franklyn Hallett Lovell Jr., the founder, coined the name klaxon from the Ancient Greek verb klazō, "I shriek".[9]

Klaxons were first fitted to automobiles and bicycles in 1908. They were originally powered by six-volt dry cells, and from 1911 by rechargeable batteries. Later hand-powered versions were used as military evacuation alarms and factory sirens. The klaxon is also remembered for its use as a submarine dive alarm in the Second World War.

The Klaxophone is a musical instrument that makes use of the klaxon's unique sound.

The English company Klaxon Signals Ltd. has been based in Oldham, England, for the last eighty years,[when?] with premises also in Birmingham. The French Klaxon company was acquired by the Italian Fiamm Group in the 1990s. In 2005 Klaxon Signals sold the rights for the hooter or klaxon range to Moflash Signalling Ltd., based in the original Klaxon Factory in Birmingham, England. The Famous Klaxet ES and A1 hooters returned home to Birmingham after ten years.[citation needed]

The Moflash Company discontinued the Klaxet hooter in 2013, but continued to produce the A1 hooter, the only original Klaxon left in production.

Several languages have either borrowed or transcribed the name into their lexicons. In Japanese, the word "klaxon" (クラクション,kurakushon?)refers to car horns in general, and in verb form (kurakushon suru(クラクションする?)) refers to sounding the horn. This is also true in languages such as French (French: klaxon), Italian (Italian: clacson), Dutch (Dutch: claxon), Russian (Russian: клаксон), Polish (Polish: klakson), Spanish (Spanish: claxon), Romanian (Romanian: claxon), Czech (Czech: klakson), Turkish (Turkish: klakson) and Indonesian (Indonesian: klakson).